A high-temperature process and a high-vacuum process are essential to form a thin film of silicon, which is an inorganic semiconductor material conventionally used in a semiconductor device. Due to the need of the high-temperature process, it is impossible to form a thin film of silicon on a plastic substrate. Therefore, it is difficult to add flexibility to a product incorporating a semiconductor device and to reduce the weight of the product. Further, due to the need of the high-vacuum process, it is difficult to increase the area of the product and to reduce the cost of the product.
In recent years, research has been actively conducted to develop an organic semiconductor device using an organic semiconductor material. Examples of an organic semiconductor device include an organic thin-film transistor, an organic thin-film photoelectric conversion device, and an organic electroluminescent (organic EL) device.
A temperature of a manufacturing process can be considerably reduced when an organic semiconductor material is used as compared to when an inorganic semiconductor material is used. Therefore, an organic semiconductor material can be deposited on a plastic substrate. Further, an organic semiconductor material has a high solubility in solvent and has a good film formation ability. Therefore, a thin film of an organic semiconductor material can be formed by a deposition method, which does not require the high-vacuum process, such as a deposition method using an ink-jet apparatus. Thus, the increase in the area and the reduction in the cost can be achieved (refer to JP-A-2009-73780).
Reported organic semiconductor materials are broadly classified into low molecular type (including oligomer) and high molecular type. For example, it has been reported that an organic field effect transistor (FET) made of pentacene, which is of low molecular type, has high mobility (refer to Yen-Yi Lin, IEEE Transaction on Electron Device, Vol. 44, No8, p. 1325 (1997), which is a non-patent document).
However, pentacene, which forms an organic semiconductor layer, has a high affinity for oxygen. Therefore, if pentacene is stored in the atmosphere, characteristics of pentacene are degraded. As a result, there is a possibility that the organic semiconductor device cannot stably operate. Further, as the temperature is higher, the degradation proceeds at a higher speed. Therefore, heat resistance is also required.